The present invention relates to contact blocks, and in particular to contact blocks for safety switches.
Safety switches are devices which are operable to selectively allow or prevent the supply of power to, for example, electrically powered kinetic machinery. A safety switch may be located at the door of an enclosure. In the enclosure may be located kinetic machinery. If the door to the enclosure is closed, an actuator may be brought into engagement with the safety switch to close a switch mechanism which allows the safety switch to conduct electricity, thereby allowing electrical power to be supplied to machinery within the enclosure. Conversely, if the door to the enclosure is opened, the actuator is disengaged from the safety switch, and this opens the switch mechanism and causes the safety switch to change from a conducting to a non-conducting state. Thus, when the actuator has been disengaged from a safety switch, electrical power is not supplied to the machinery within the enclosure.
Whether the safety switch is in a conducting or non-conducting state is determined by the configuration of electrical contacts located within the safety switch. These contacts are located within a contact block. A prior art contact block 1 is shown in FIG. 1a. 
FIG. 1b illustrates certain parts of the contact block 1 of FIG. 1a. FIG. 1c shows these parts in cross section, taken across the plane P1 and in the direction of the arrow A1. FIGS. 1b and 1c are referred to in combination. Four pairs of fixed electrical contacts 2 are provided and fixed in position relative to the contact block 1. Extending along the contact block 1, and in-between the pairs of fixed contacts 2, is a contact block plunger 3. The contact block plunger 3 is provided with four bridging contacts 4 which extend through the body of the contact block plunger 3 and protrude from the sides of the contact block plunger. The bridging contacts 4 are moveable, within limits imposed by the body of the contact block plunger 3, back and forth along the length of the contact block plunger 3. The contact block plunger 3 is moveable in the contact block to bring the bridging contacts 4 into contact with specific pairs of fixed contacts 2. When the bridging contacts 4 are brought into contact with the pairs of fixed contacts 2, a current may flow between the pairs of fixed contacts 2.
The contact block plunger 3 is biased to a default position by a first helical spring 5. The first helical spring 5 runs alongside and is substantially parallel to the main body of the contact plunger 3, and is fixed to and exerts a pulling or pushing force against a lip 3a of the contact plunger 3. The lip 3a extends in a direction substantially perpendicular to the length of the contact block plunger 3. In a similar manner, each bridging contact 4 is biased to a default position by one of a number of second helical springs 6 which are located in and extend along the contact block plunger 3.
In use, the contact block plunger 3 may be moved in any appropriate manner. For example, another element (not shown) may push against an end of the contact block plunger 3 to move the bridging contacts 4 into and out of electrical connection with the fixed contacts 2. In many safety switches, a cam arrangement is provided, the cam arrangement being rotatable by engagement with an actuator (e.g. a key). A cam follower (sometimes referred to as a cam plunger) is provided which is biased against a surface of the cam arrangement. First, when the cam arrangement is rotated, the position of the cam plunger may be changed. The cam plunger's position may be changed to push against or pull the contact block plunger 3 to move the bridging contacts 4 into and out of electrical connection with the fixed contacts 2.
Although the prior art contact block 1 shown in FIGS. 1a to 1c is used in large numbers and in a variety of different types of safety switches, it has a number of disadvantages. Firstly, it is possible that the contact block plunger 3 can move towards and away from the fixed contacts 2. This may cause the bridging contacts 4 to be misaligned with the fixed contacts 2 which may mean that it is difficult or impossible to reliably make and maintain an electrical connection between the fixed pairs of electrical contacts 2. Similarly, the contact block plunger 3 may move up or down (in relation to the orientation of the contact block as shown in the Figures), which again may cause misalignment with the fixed contacts 2. When the first helical spring 5 is compressed, it may ‘snake’, such that the first helical spring 5 when compressed does not extend in a linear fashion, but in an undulating or wave like fashion. If the first helical 5 spring does this, its biasing force will not act solely in an axial direction (i.e. parallel to the length of the contact block plunger 3) but also in other directions. This may cause the contact block plunger 3 to be pushed into parts of the contact block 1 (for example, the fixed pairs of contacts 2) or to be pushed away from parts of the contact block 1. In either case, this could again cause misalignment between the bridging contacts 4 or the contact block plunger 3 and the fixed contacts 2. If pushed into parts of the contact block 1, the contact block plunger 3 could also suffer from increased wear due to increased friction between itself and the parts against which it is pushed. The fixed pairs of contacts 2, for example, may become loose if consistently and/or repeatedly pressed against by the contact block plunger 3.
In some safety switches, the contact block 1 is connected to a printed circuit board (not shown). Electrical connection is made to the printed circuit board by a terminal block or terminal block connector (not shown) which is attached to the printed circuit board. However, if wires connected to the terminal block are pulled, the terminal block may be pulled off the printed circuit board. Furthermore, the printed circuit board may become damaged and require replacement.
It is therefore an object of the present invention to provide a contact block and contact block plunger which obviate or mitigate at least one of the disadvantages of the prior art, whether identified herein or elsewhere.